Organic Light Emitting Displays (OLEDs) are widely seen as the next generation display technology that will come to replace existing display technology. Amongst the various challenges to be dealt with in the development of OLEDS, effective encapsulation remains one of the most significant.
One commonly known problem with OLED structures and other oxygen and/or moisture sensitive electroluminescent devices is that they degrade rapidly under atmospheric conditions. In order to protect them from degradation, various types of encapsulation have been used to isolate the electroluminescent devices from the environment. It is estimated that for an OLED to achieve reliable performance with a lifetime exceeding 10,000 hours, the encapsulation around the reactive electroluminescent material should have an oxygen transmission rate (OTR) less than about 5 to 10 cc/m2/day and a water vapor transmission rate (WVTR) of less than about 10−5 g/m2/day at 39° C. and 95% RH. The technical challenges brought about by these stringent requirements have driven constant developments in encapsulation technology over the years.
Conventional encapsulation structures comprise a substrate on which the electroluminescent device is formed, and a covering structure which seals the electroluminescent device against the substrate. In certain types of electronics applications, such as hard disk drives, one approach in improving the barrier properties of an encapsulation is to utilise thick, gas impermeable encapsulation structures. However, this approach is not suitable for applications such as OLEDs or solar cells in which opacity is a requirement and the quality of light transmitted through the encapsulation must be maintained.
Recent developments in OLED technology has seen the rise of flexible OLEDs which require that the encapsulation structures are flexible, thereby making it more apparent that encapsulation technology has not kept pace with developments in OLED technology. The substrate for a flexible OLED should ideally combine the gas barrier properties, chemical resistance and surface properties of glass with the flexibility, toughness and processability of conventional plastics. Transparent polymers were used to form various parts of an encapsulation structure because they were inexpensive and easily processed. However, due to their permeability to moisture and oxygen, encapsulation structures formed from polymers alone are nowadays considered to be inadequate for achieving low permeation rates as the required standard for oxygen and water vapor impermeability are orders of magnitude lower than what is achievable with the best polymer substrates using today's state of the art in industrial polymer technology.
More recently, barrier laminates derived from certain types of inorganic materials were found to have better barrier properties than polymeric barrier laminates. Metals such as aluminium are now used as barrier materials (e.g. aluminium foils) for packaging food substances and pharmaceutical drugs. Despite possessing improved barrier properties, it has been found that the performance of inorganic barrier laminates is still limited by inherent structural defects. Recent studies have shown that structural defects such as pinholes, cracks, grain boundaries, etc., allows oxygen and moisture to permeate over time, leading to poorer than expected barrier performance. It is difficult to control fabrication to such an extent that defects are completely eradicated because such defects are randomly formed, independent of the method of fabrication.
One approach that has been used to overcome the problems of poor barrier properties in polymer barrier stacks and the problems in forming defect-free inorganic barrier layers is to stack multiple polymer/metal oxide layers together to form a barrier stack. It has been found that combining polymer layers with metal oxide layers enables the defects of one polymer/barrier oxide stack to be decoupled from the next polymer/barrier oxide stack, thereby slowing down the propagation of oxygen/moisture from one inorganic layer to the next.
Vitex Inc. discloses in U.S. Pat. No. 6,866,901 a multilayer barrier stacks comprising multiple sputter-deposited aluminium oxide inorganic layers separated by polymer multilayers (PML) comprising organic polymers. This multi-layer barrier stack design is based on the principle of decoupling the defects of two successive barrier oxide layers in the multilayer barrier stack. A recent modelling study suggests that defect decoupling due to the organic/inorganic multilayers forces a tortuous path for moisture and oxygen diffusion, thus reducing the permeation rate by several orders of magnitude.
Despite this development, a large number of thick barrier oxide and polymer interlayers are needed in order to achieve ultra high barrier properties of better than 10−6 g/m2/day, or even better levels of 10−6 g/m2/day. Variations in overall barrier performance still arise due to factors such as whether the pinholes in one layer are lined up with the defects in the other layers. Other limitations of the multilayer stack approach is that it suffers from poor adhesion and frequent delamination occurs, especially during the thermal cycles of the OLED fabrication processes, since the inorganic and organic layers have sharp interfaces with weak bonding structure due to nature of the sputter deposition and PML formation processes. It also results in thick panels with poor transmission qualities and which cracks easily.
HELICON Research, Inc. discloses in US Patent Application No. 2005/0051763 an organic/inorganic nanocomposite structure formed by infiltration of a porous inorganic layer by an organic material. The composite structure is produced by vacuum deposition techniques. In contrast with the aforementioned techniques, this document teaches the fabrication of porous inorganic barrier layer onto plastic substrate and then depositing organic material in the barrier layer such that it infiltrates the porous inorganic material to form a continuous layer.
General Electric Inc. discloses in EP 1 164 644 a barrier system which utilizes the high temperature resistance and high clarity of transparent Lexan™ film properties to enable a 125-micron-thick substrate to withstand the heat involved in OLED fabrication and to allow optimal light transmission from the device. The barrier coating comprises silicon oxide compounds which are applied onto the substrate using plasma enhanced chemical vapour deposition. The barrier coating prevents degradation of the device from oxygen, moisture, chemicals, and electronic conductivity while promoting light transmission. Additionally, nanoparticles reactive with moisture are incorporated into the base substrate.
Nanocomposite barrier materials comprising mineral clay nanoparticles distributed in a polymeric binder have been developed for use in food packaging materials. For example, U.S. Pat. No. 5,916,685 discloses a multilayer barrier laminate comprising an exterior polymeric layer containing non-reactive clay nanoparticles in the quantity of about 0.1 to 10% by weight of a polymer layer in which it is distributed. The polymer layer is arranged on an inner metal oxide barrier layer. The barrier laminate achieves a water vapour transmission rate of about 0.61 g/m2/day over a 24 hr period, and is clearly unsuitable for OLED encapsulation.
Accordingly, limitations in the barrier performance of existing encapsulation structures still exists. An object of the present invention is to provide an alternative barrier stack that has improved barrier properties and is inexpensively fabricated.